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No lead in unglazed tiles?

March 9, 2011 - 8:42am

yankeedave

No lead in unglazed tiles?

I know this has been discussed before, but I am having a hard time finding a definitive answer. If I buy unglazed quarry tile, of unknown origin and content, do I need to worry about lead or other harmful substances? I'm beginning to think that all the worries about lead are stemming from warnings about glazed tile, which have led some people to think that the tile itself is the problem, whereas the actual source of the lead is the glaze, not the tile. If so, then just about any unglazed tile should be lead-free. But before I run out and buy some I'd like to see if anybody has any information to corroborate or call into question my theory.

I went to a local flooring shop and bought a chipped piece of stone flooring that they couldn't use. Half an inch thick, they were glad to give it to me for a few dollars. They even cut it to size for me.

=== I know this has been discussed before, but I am having a hard time finding a definitive answer. If I buy unglazed quarry tile, of unknown origin and content, do I need to worry about lead or other harmful substances? ===

In short, yes. I've worked both for a large refractory manufacturer and for a consumer products company that sold to big-box stores, and I would strongly recommend that you stick with finished products that are marked NSF (trade mark for kitchen safe) or bulk products for which the seller will give you a complete spec sheet and MSDS and which are listed for use in, e.g., ovens.

The cost pressure on big-box stores is incredible, both for their suppliers and internally for their "sourcing" groups. Every large retailer has its specs and safety requirements, but if you are selling them tile, the industrial ceramics company down the road calls and asks if you want to dispose of a trainload of rejects for them, and its only for garden footstones after all... In situations like that questions don't get asked and products which were reasonable safe if used as intended (the inside of a blast furnace has its own safety issues regardless of what is in the lining) can deliver nasty surprises when used other than as intended.

And regardless of where we personally buy things the structure of the big-box stores is now affecting the entire economy; your specialist retailer can be affected by what his suppliers are doing to serve Giganto-Mart without him even being aware of it.

sPh

Fortunately the consumer products company I worked for ensured that its products met its specs and were safe (some were NSF listed depending on application).

Well, for one thing unglazed quarry tiles are an increasingly rare sight in home improvement store. I, for one, have not seen them in local stores for well over a year, may be even two. As to your question, nobody really knows. It is not unreasonable to assume that all it is is fired clay, but I don't think even manufacturer scrutinizes the composition of said clay - after all these things are not sold as food grade material. Personally, I feel that baking on a parchment would provide a sufficient barrier against anything in the tiles, after all remember they have been fired at the temperatures exceeding what we have in our ovens by well over 1000 degrees. I don't bake on tiles though.

If one of the advantages of using a stone is that its porous composition allows for steam to escape from the bottom of the loaf or pizza, doesn't using parchment paper tend to counteract that? I mean if I slide a loaf of bread onto a stone, on a piece of parchment paper, am I really gaining anything compared to sliding it onto a baking sheet, on a piece of parchment paper? I know the stone will hold heat longer than a metal sheet, but that's a different issue.

Various authors suggest that a pizza stone sucks moisture, in the form of steam, away from the dough as it bakes, because a pizza stone is porous and can absorb moisture. Parchment paper would seem to me to create a barrier that would not allow that to happen.

Seriously? I've never seen suggestions of this sort. They don't make any sense anyway. Think about it this way. As you noted, at the temperatures used in baking water occurs in its gaseous state, that is steam. Now, the volume of your typical 14" diameter, 0.5" thick, I'll spare you the calculation but the volume occupied by the stone can hold sligthly less than 1 gram of steam. In reality, since most of that volume is actually occupied by the stone itself, we are talking about a small fraction of 1 gram of steam that can theoretically be absorbed.

I have seen this suggested -- that parchment paper forms a barrier that reduces the release of moisture -- but I have no idea if there is any truth to it. I usually use parchment when making pizza, but I'll often try to slide it out after 5 minutes. Does it help? Shrug.

I've done the same. I'm beginning to think, if the oven's at 550 and you've got a preheated, flat surface for the dough, it's probably not going to make a tremendous amount of difference whether it's stone, aluminum, or otherwise. I did find this experiment but the guy didn't test a stone, so it's of limited value. The best approach for me may be to stop worrying about it, frankly.

You may be absolutely right but I am surprised you have never seen such a claim before.

"The porous nature of the stone used also helps absorb moisture, resulting in a crisp crust." - Wikipedia

"The pizza stone will help maintaining an even cooking temperature and absorbs moisture so your pizza crusts are always nice and crispy." - www.bialettishop.com

"In essence, the stone both causes, and then absorbs, the steam trying to escape from the dough. Again, less moisture, more crispy. Soild petal pizza pans give the water nowhere to go, so the water is either retained in the dough, or must make its way to the surface of the pie before escaping." - Breadtopia.com

I could go on.

Now maybe that's all b.s., but that seems to be a large part of the rationale for using a stone. If the only good reason for a stone is to add thermal mass, I could lay some bricks on the oven rack and place a baking sheet on top of them.

Look, how can you possibly trust Wikipedia here? Look at the expert who wrote that. Again, seriously? That's who we get to rely on? Ditto on the internet store, copywriters are in some respect a similar to wiki people, they are often just as ignorant, but at least they have a valid excuse for doing things they do - they're being paid for that. As to the Breadtopia link... what is "soild petal" anyway? That just sounds nasty.

To make good pizza you have to meet two conditions. First, you have to generate a large amount of heat. Second, you have to transfer it to your dough, and do it rapidly. So in essence what you need is an oven that has high maximum temperature (550 doesn't quite cut it) and yes, thermal mass - something with large heat capacity and decent thermal conductivity. This water absorption business is just what you said - bs.

I didn't say I trusted any of these sources. I gave them as examples of a rationale for using pizza stones that you apparently have never heard of, but which is quite widespread. If you're that worked up over it then you'd better start logging on to these sites to dispel this pernicious myth.

"I am pretty sure I am absolutely right" Maybe (I'm no authority), but I must say there's more than one person who's always believed that moisture absorption is important to a crispy pizza crust; I for one am not confident enough to conclude "B.S." without any doubt. True, baking stones can't be absorbing very much, as they're obviously not built like a sponge. But it doesn't take very much.

Another way to produce crisp pizza crust is to use a pan with a fine grid of holes in it, which I initially didn't expect to make a whole lot of difference. But it's enough to avoid "soft" (or even "soggy") crust.

Getting back to the original question - if you buy unglazed tiles from an unknown source, my opinion (and it's only that - and opinion) is, yes, there is reason to be concerned about food safety there, for many reasons.

From my own gardening experience (long time ago - I've been in apartments for a long time), I read to have my soil tested. I lived in a city at the time, on a busy street, and I discovered that the soil could contain lead due to exposure to car fumes before lead was outlawed. That's just from cars - thankfully I didn't have to be concerned about things like the dumping of toxic material where I gardened, etc...

Now since the unglazed tiles sold here in the US in home improvement stores aren't produced for food use, and since they are made in areas where poverty and the lack of environmental controls go hand in hand - well, for me, it's not worth the risk.

That sounds sensible. It also gives me the idea to take a sample of a tile to my local agricultural cooperative extension to see if they can test it. If I can't be sure I'll stick with baking sheets. Despite my best efforts, my pizza stones keep cracking and I'm tired of replacing them.

Great idea - to see if your local agricultural coop extension office can test them. If they say no, I'd ask if getting some ground up might help. After all, they started out as dirt, so they should be able to. Don't know how easy/difficult that might be to do (smashing them to bits). Might depend on how angry you are at the time, lol.

If they do it, keep us informed. I never even thought of asking them!

I think I'm going to try the steel idea. I live in an area where there should be a lot of places that might have some, cheap. Just waiting for the weather to warm up, as I'm tending to hibernate more than usual (pain, getting old, yadda yadda yadda).

I'm an artist and drawing on when I was in art school lead was never used to lead for anything but glazes. In art school you are taught to be careful about exposing yourself to hazards with a wide variety of substances. I know this is beating a dead horse but I use a kiln shelf that I bought to use for that purpose. Meaning that it has never used in my kiln only in my kitchen. They don't tend to crack since they are designed to be use at temperatures way beyond what kitchen oven can reach. Pottery supply will have a variety of sizes and they are pretty easy to find every where. I paid about $25 for mine and it's 1/2 inch thick.

thanks, I never would have thought of that. They look to be about the same price as a pizza stone but if they last longer it would be worth it. I've tried to be careful with my pizza stones but my third one just cracked.

In order for lead to be poisonous, it has to be ingested. If I am not mistaken, lead in glazes is only a problem if the glaze is crumbling or being chemically attacked by something that is then eaten, i.e. strongly acidic liquid food. If all you are doing is baking bread on intact ceramic glazed tiles, I see no hazard in using them. The likelihood of ingesting a signficant amount of lead, if any at all, has to be slim to none in baking.

I use the heaviest glazed 16" porcelain tiles that I could get my hands on ($4-10 ea); I have no idea if they are lead free or not. I stack them two high for about 14-15 lbs (IIRC) total.

the transfer of lead, if at all present, cannot be significant. The form in which the lead is present is not reactive enough with the acids that may be present in a bread to be a problem. Additionally, while many breads are highly hydrated, they are not in contact with the surface of the tile in this state to facilitate a reaction with the lead--as unfavorable as that would be anyhow. Reactions where the materials in question are in the solid state or nearly so are typically very slow, especially compared to ones where the reactants are all in solution (dissolved). The rate at which the very weak acids (lactic and acetic) in the bread would diffuse out and into the tile, let alone, react, are so slow as to be virtually nonexistent.

Lead poisoning is a hazard only when the lead can be easily extracted and ingested, e.g. eating or breathing leaded paint chips or dust, cooking very acidic food at elevated temperatures. Stomach acid (hydrochloric) is several orders of magnitude more reactive than acetic or lactic acids towards the typical lead compounds that might be found in a glaze or tile.

Intact high fired glazed porcelain tiles are not a significant lead hazard when baking bread.

I recognize that this may be a contentious issue. I hope that by considering the underlying chemistry that your fears can be allayed.

Edit, to add: I would not recommend (of course) the use of leaded glazes for cooking liquid materials especially under high heat. There is ample evidence of leaching under those conditions.

Also, note that lactic, acetic and citric acids, the only components, as far as I know that might react with lead, will either boil off and or decompose at bread oven temperatures.

I bought one unglazed tile and one travertine tile and am having them professionally tested for $15 apiece. I figure the expense is worth it in the long run both healthwise and because they are a lot cheaper than pizza stones. If I can use either, I plan to put clay bricks in the oven to add thermal mass, since these are a little thinner than most pizza stones, though they're still fairly heavy. It may not be quite the same as using a pizza stone but it ought to be close.

I'd be happy to post the results, but I think this will remain an ongoing debate, since the tests would only relate to these particular stones (I'll try to get the brand name if I can). And they're only being tested for lead; I suppose there could theoretically be any number of any other nasty substances in there. But if they come back lead-free, I'll run that risk, since I have no reason to suspect the presence of such substances to begin with.

I appreciate the scientific basis for lead not being an issue, but on an emotional level I'd probably still be afraid to use a stone containing lead. It may just be an emotional response but that's how it is.

I bought a used double wall oven off of craigslist that I am going to set up in my basement exclusively for bread baking (I've got the bug...). Instead of tiles, which I currently use, I am going to go to one or two of the local marble countertop places around here and see if I can get them to cut me some marble or granite scraps or seconds to size. I've heard about other people doing this, supposedly for $20-30. The scrap is a process loss for them and the cutting is trivial--it doesn't have to be a cosmetic beauty. The marble will be more massive than the tiles and should better utilize the space. I'll try to remember to follow up and let everyone know how I made out in a few weeks.

That's what I did. I have a slab of marble flooring that I got from a flooring contractor. It was chipped on one side, left over from a job. He cut it and gave it to me for $10. I brought him some bread. The oven spring is terrific. I don't understand all the fuss about tiles and lead. Get stone flooring.

Sounds good, but as with most of this stuff, if you keep reading these kinds of posts you'll find concerns raised. I've seen suggestions that marble won't hold up well to oven temperatures and even that it can shatter. I don't know if that's actually happened to anybody or if its just speculation. And marble and granite are not porous the way that clay is, which again may or may not make a whit of difference in the final product, depending on whom you believe. But if it's working for you I guess that's better than anybody's abstract theory.

You're right, marble has its faults; I was thinking about all the stone countertops I see and should have said granite. It has good thermal stabilty and is inert at the temperatures we bake at and much higher.

Granite may not be very porous, but I think that at temperatures whiich are well above the boiling point of water, absorption is not a big issue.

I have been baking on glazed tiles with great results and they are not porous. Also, if you dig around there are other posts of people having good luck with stone.

The seller likely either won't know what you're talking about, or will pretend to not understand you, or won't be able to express an answer in comprehensible terms. Sometimes you can tell by the color, but not always. So the only realistic course of action is to test it yourself.

If you don't want to google for ways to test the porosity of granite, just put a drop of water on the granite. If it's still there after five minutes, you should be able to use that granite in your oven. But if it's soaked in after five minutes, avoid baking stone use of that granite.

(Also beware of "sealed" granite. Most sealers expect the granite to be used as a countertop. So mostly they're not toxic, and they repel water and stains quite well [a typical sealer is beeswax]. They'll make any granite pass the "water drop not soaking in" test easily (or to say the same thing less positively, they'll fool the test so well it becomes useless). But many sealers will not stand up to heat.)

It used to be common to use marble as the surface for rolling out piecrusts, kneading bread dough, and so forth, because marble was the best available at that time [nobody knew how to smooth granite at a reasonable cost yet]. But marble is no longer the best material for a cool work surface, and it definitely is not a recommended material for use as a baking stone.

yes, most natural prodiucts are variable. Only one side of a scrap piece is likely to be finished, so a water test should be possible. I've got to believe that any piece of granite is not going to absorb much water at 450+F. Maybe some grease, but no water. If all I use it for is bread, most pieces should fine.

I used to work in the coatings industry and, best I recall, as others have said, the stone finishes are either acrylic, a vinlyl emuslions (like clear latex paint) or maybe a polyurethane or silicone. The silicones have better high temperture stability than the former, but it's best to avoid all of them.

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